GB2569366A

GB2569366A - Solid sorbent reactor

Info

Publication number: GB2569366A
Application number: GB1721034.5A
Authority: GB
Inventors: Strand Asbjørn; Ranjan Rout Kumar; Chen De
Original assignee: Fjell Biodry As
Current assignee: Fjell Biodry As
Priority date: 2017-12-15
Filing date: 2017-12-15
Publication date: 2019-06-19
Anticipated expiration: 2037-12-15
Also published as: GB201721034D0; US11759746B2; CN111712314A; WO2019115831A1; GB2569366B; EP3723891A1; US20210093993A1; GB201817072D0

Abstract

A mass transfer system for providing mass transfer between a gas and a solid reactant, the mass transfer system comprises a gas inlet (14, Fig 6), a gas outlet (13, Fig 6), a reactant inlet 5, a reactant outlet 10, at least one mass transfer region 7 arranged between the reactant inlet and the reactant outlet, a first gas chamber and a second gas chamber. The second gas chamber is different from the first gas chamber preferably occupying a different vertical position. In use, solid reactant such as calcium oxide (CaO) is retained preferably by strainer plates, within the at least one mass transfer region as the solid reactant moves through the mass transfer region and the mass transfer between the gas and the solid reactant occurs in the mass transfer region. In use the first gas chamber, second gas chamber and mass transfer region are arranged such that there is a flow path for gas from the gas inlet to the gas outlet that includes gas flowing from the first gas chamber into one of the one or more mass transfer regions, the gas then flowing from said one of the mass transfer regions into the second gas chamber and the gas then flowing from the second gas chamber back into said one of the mass transfer regions. The system provides for an efficient reactor for use in large scale carbon capture and storage (CCS) systems.

Description

Field

The field of the invention is the design of reactors for reactions between a gas and a solid. Embodiments provide a new reactor design with a number of advantages over known reactor designs.

Background

Fossil fuels provide a significant portion of the world’s energy needs. A problem with fossil fuel combustion is that it is a major source of anthropogenic carbon dioxide (CO2) emissions.

A known technology for reducing CO2 emissions into the atmosphere is carbon capture and storage (CCS). The three main options for capturing CO2 from fossil fuel plants are post-combustion, pre-combustion, and oxy-combustion. An important advantage of postcombustion technologies is that the technology can be retro-fitted to existing power plants.

In a CCS system, a sorbent removes CO₂ from a carbonaceous gas. The CCS system also comprises a sorbent regenerator in which the sorbent releases CO2 into a controlled environment so that the CO₂ is not released into the atmosphere. The regenerated sorbent is then re-used to remove CO2 from gas. The sorbent is therefore moved around the CCS system in a loop.

The sorbent used for post combustion CO₂ capture can either be any of a number of commercially available aqueous amine solvents or a sorbent based adsorption technology. An advantage of absorption processes that use amine solvents is the fast kinetics in the absorption reactor. However, disadvantages include high capital and operating costs. The use of amine solvents can also cause environmental problems. The use of solid sorbents for CCS has a number of technical and economic advantages over the use of amine solvents. An example of a solid sorbent for CCS is calcium oxide (CaO).

For realisation of a solid sorbent based CCS system, a gas-solid reactor is required for supporting the reaction between a solid sorbent and carbonaceous gas, e g. a flue gas from a fossil fuel power plant. Three types of beds are typically used in gas-solid reactors. These are fixed beds, fluidized beds, and moving beds 7.

When reactors with fixed beds are used in a system that is operated continuously, the reactors require complicated operation and control procedures in which the bed is alternatively saturated and regenerated in a cyclical manner. Another problem with fixed bed reactors, in the specific application of CCS applied to flue gas from fossil fuel fired plant with a CaO sorbent, is that the volume of flue gas is three orders of magnitude larger than the optimum volume flow of solid particles. Due to limitations on gas velocity to prevent the bed from fluidizing, a large number of fixed beds are needed and this greatly increases the capital cost.

A fluidised bed of a reactor is a bubbling and circulating bed in which solids and gas are well mixed. The mixing ensures good heat and mass transfer characteristics. The fluidized bed reactor is an effective mixing device for solid particles due to the large flows inside the reactor. However, a problem with fluidised bed reactors is that the retention time of individual solid particles has a very wide probability distribution. Some particles can stay in the reactor for seconds, whereas other particles may stay be in the reactor for minutes. When the optimum retention time for the reaction is in the order of a few minutes, a significant proportion of the solids will be in the reactor for too short a time for the reaction to be effectively completed, and another significant proportion will still be in the reactor long after reaction is completed. This reduces the efficiency of fluidized bed reactors and reduces their economic viability. Another problem with the use of fluidized bed reactors is that there can be significant attrition of sorbent particles and erosion of the reactor vessel and internal components.

In known designs of moving bed reactor, a solid particles are contained in a vertically oriented reactor chamber. Solid particles are continuously fed into the top of the reactor and taken out from the bottom of the reactor in a controlled manner. By the act of gravity, the solid bed inside the reactor moves from the top of the reactor to the bottom of the reactor as a plug. This secures a specific retention time of solid particle passing through the reactor. The solid particles are fairly densely packed in the moving bed, leaving a relatively small volume for the gas phase. Gaseous reactants can pass through the reactor in a co-flow, counter flow or cross flow manner. Known designs of moving bed reactor are problematic when the gas has a relatively low concentration of reactants such that there is a large proportion of inert gas inside the reactor that needs to be transported through the solid bed. This creates large pressure drops in the gas phase, and in the case of counter flowing gas, the gas may quickly start to fluidize the solids bed so that the system does not possess the characteristics of a moving bed reactor.

There is a general need to provide an efficient reactor for use in large scale CCS applications.

Summary of invention

Aspects of the invention are set out in the independent claims.

Preferable aspects are set out in the dependent claims.

List of figures

Figure 1 is a cross-section through a reactor design according to an embodiment;

Figure 2 is a cross-section through a reactor design according to an embodiment;

Figure 3 is a cross-section through a reactor design according to an embodiment;

Figure 4 is a cross-section through a reactor design according to an embodiment;

Figure 5 is a cross-section through a reactor design according to an embodiment;

Figure 6 is a cross-section through a reactor design according to an embodiment;

Figure 7A is a cross-section through a reactor design according to an embodiment;

Figure 7B is a top down view of a reactor design according to an embodiment;

Figure 7C is a cross-section through a reactor design according to an embodiment;

Figure 7D is a cross-section through a moving bed of a reactor design according to an embodiment;

Figure 7E is a cross-section through part of a loop valve of a reactor design according to an embodiment;

Figure 7F is a cross-section through a loop valve of a reactor design according to an embodiment; and

Figure 8 is a CCS system according to an embodiment.

Description of embodiments

Embodiments provide an efficient reactor for use in a CCS system for large scale CCS applications. The CCS system preferably uses CaO particles as a sorbent. The CCS system preferably also comprises a sorbent regenerator and means for looping the sorbent around the CCS system.

In the application of CO2 capture from flue gas from a natural gas fired combined cycle power plant (NGCC), the concentration of reactant is less than 4%vol. A typical 400 MW class NGCC produces nearly 2000 m³/s exhaust from the gas turbine. To capture the CO2 by carbonate looping in a moving bed reactor requires a flow of approximately 400 kg/s of CaO particles. Effective CaO particles would need to be in the form of substantially spherical pellets of diameter 0.5mm to 1 mm having a bulk density of 2000 kg/m³. This results in a volume flow of circulating solids of 0.2 m³/s. An acceptable utilization of the CaO pellets requires a retention time in the reactor of approximately 4 minutes. That implies a bulk volume of the solid pellets bed of 48 m³. It is clearly not possible to pass 2000 m³/s exhaust gas through a known design of moving bed of such volume without causing extreme pressure drops or fluidization of the bed.

For a coal fired power plant, the CO2 concentration in the gas may be 12-14%vol. Flue gases from industrial process like a blast furnace for steel production or a cement kiln can have concentrations of CO₂ above 20%. However, the inert gas volume left after the CO₂has reacted with the solids has practically the same volume. Known moving bed reactor designs therefore experience similar problems to those described above.

Embodiments solve the above problems by providing a new design of moving bed reactor for supporting gas-solid reactions. The reactor comprises a plurality of moving beds 7 for transporting solid sorbent through the reactor. The reactor also comprises a plurality of gas ducts for gas flows through the reactor. The gas flows are controlled such that gas is forced to flow across one or more moving beds 7 a plurality of times.

A particularly preferred application for the reactor is supporting the reaction between a solid sorbent and a carbonaceous gas in a CCS system. The sorbent is preferably CaO particles in the form of substantially spherical pellets/particles with a 0.5mm to 1 mm diameter and a bulk density of 2000 kg/m³.

The reactor according to embodiments is described in more detail below.

Figures 1 to 6, 7A and 7C show cross-sections through a reactor according to an embodiment. Figure 7B is a top down view of the reactor. As shown in Figures 1 to 6, 7A and 7C, at the top of the reactor is an inlet 5 through which sorbent enters the reactor. At the bottom of the reactor is an outlet 10 through which the sorbent exits the reactor. Between the inlet 5 and the outlet 10 is a main body of the reactor.

The main body of the reactor has outer walls 1. At the top of the main body of the reactor is an upper bed 6. Provided below the upper bed 6 are a plurality of moving beds 7 that extend vertically downwards through the main body of the reactor to a lower bed 9 at the bottom of the reactor. The main body of the reactor also comprises a gas inlet 14, through which gas enters the reactor, and a gas outlet 13, through which gas exits the reactor. Between the vertically arranged moving beds 7, as wells as the outer walls 1 of the main body, are gas ducts for gas flows in the main body.

The walls of the moving beds 7 comprise strainer plates 3. The strainer plates 3 have the property of retaining the solid sorbent within each moving bed 7 but gas is able to pass through the strainer plates 3. A possible design of the strainer plate 3 may be types wedge wire screens, such as those manufactured by Intamesh, see http://www.intanjesh.co.uk/ as viewed on 13^th December 2017. It is known to use such screens as strainers and filters in other industries than the field of embodiments, such as in shale shakers for mud and cuttings separation during the drilling of oil and gas wells. Figure 7D is a cross-section through a moving bed 7 with wedge wire screens as walls according to an embodiment.

In order to minimise the friction and stress on the particles of sorbent, the strainer plates 3 are preferably orientated vertically. For the same reason, when the strainer plate 3 is a wedge wire screen, the flat side of the wedge should provide the inner surface of the outer wall of the moving beds 7, as shown in Figure 7D. The wedge should have the property that the openings in the wedge are not so large that particles of the sorbent can pass through the openings, but the openings should be large enough to allow gas to pass through them. The diameter of the sorbent particles/pellets will typically be larger than 0.5 mm and an opening distance in the wedge wire screen of between 0.2mm to 0.4mm would therefore be appropriate.

Alternatively, the strainer plates 3 may be provided by perforated plates, or a thick rigid perforated plate with fairly large diameter perforation (approx. 10 mm) cladded with a thin sheet with very small perforation (<1 mm). In some applications these may be sufficient for retaining particles of solid sorbent in the moving beds 7 and less expensive than wedge wire screens.

The gas ducts comprise horizontally arranged baffle plates 2 and gas is unable to flow directly through a baffle plate 2. Preferably there is at least one baffle plate 2 in each gas duct 4. The provision of one or more baffle plates 2 in each vertically aligned gas duct divides each gas duct into a plurality of separate and vertically aligned chambers.

As shown in Figure 6, when a gas flow in a gas duct 4 reaches a baffle plate 2, the gas is forced by the baffle plate 2 to flow out of its current chamber in the gas duct, through a strainer plate 3 and into a moving bed 7. The gas then flows across the moving bed 7 and into a chamber of a different gas duct. In order for it to be possible for gas to flow from a chamber in a first gas duct 4 to an adjacent chamber in the same first gas duct, via a chamber in a second gas duct, the vertical position of baffle plates 2 in adjacent gas ducts is preferably staggered as shown in figure 6. That is to say, in any two adjacent gas ducts 4, all of the baffle pates 2 have different vertical positions. As shown in Figure 6, a gas path through the reactor may comprise flowing across the same moving bed 7 plurality of times. Clearly, a gas flow path through the reactor may additionally, or alternatively, comprise flowing across a plurality of different moving beds 7.

The gas flow paths from the gas inlet 14 to the gas outlet 13 therefore comprise gas flows through a plurality of chambers, with the gas flowing through one of the moving beds 7 whenever it flows between two chambers.

In a preferred embodiment, the main body of the reactor is substantially a rectangular cuboid. Each of the moving beds 7 and gas ducts are also substantially rectangular cuboids. Two walls of each moving bed 7 are provided by strainer plates 3, two further walls of each moving bed 7 are provided by parts of the outer wall 1 of the main body and the moving beds 7 are open at each end in order for sorbent to enter and exit the moving bed 7. Each gas duct is also cuboid and are either formed between two moving beds 7 or between a moving bed 7 the outer wall 1 of the main body. The baffle plates 2 in the gas ducts are also thin rectangular cuboids. Advantageously, the components of the main body of the reactor all have a rectangular cuboid construction and can therefore be easily made.

The construction of the reactor is also easier when rectangular cuboid components are used.

Although the main body of the reactor is preferably rectangular cuboid, embodiments also include the reactor being cylindrical, as well as other shapes.

According to a preferred embodiment, the rate at which sorbent passes through the moving beds 7 can be controlled. At the lower end of each of the moving beds 7 is an exit duct 8. Each of the exit ducts 8 comprises a flow control mechanism. The flow control mechanism may be, for example, a loop seal or adjustable baffles and allows the rate at which the sorbent moves into the lower bed 9 to be controlled. The loop seal may be as shown in Figure 7E and 7F. Preferably, some of the gas is fed into a gas inlet of the loop seal. This generates an up-flow of gas that reduces that rate at which sorbent particles move through the moving beds.

The reaction between a carbonaceous gas and CaO is an exothermic reaction. It is therefore necessary to remove heat from the reactor in order to maintain the conditions of the reaction between the gas and the sorbent within a desired temperature range over a long period of use, or continuous use, of the reactor. In order to remove heat for the reactor, the reactor preferably comprises one or more cooling tubes 12. As shown in figures 4, 5, 7A and 7C, a cooling tube 2 has an inlet 15 into the reactor and outlet 16 from the reactor. The cooling tube 2 is arranged to pass the through one or more of the gas ducts.

Embodiments include there being one or more cooling tubes 12 for each chamber of a gas duct, one or more cooling tubes 12 for each gas duct or one or more cooling tubes 12 for all of the gas ducts. The cooling tubes are arranged to directly cool the gas in order to remove heat from the system. Since the cooling tubes 12 do not pass through the moving beds 7 they do not impede the movement of the sorbent in the moving beds 7. Within each cooling tube is a circulated coolant in a heat exchanger arrangement according to known techniques.

Figures 3 and 5 show gas inlet ports 312 and gas outlet ports 311 for providing a gas flow into the gas ducts 4 of the reactor. There is a first manifold between the gas inlet ports 312 and the gas inlet 14, and a second manifold between the gas outlet ports 311 and the gas outlet 13. In the shown configuration with the gas inlet 14 below the gas outlet 13, the relative flows of the gas and sorbent through the reactor comprise a counter flow component in addition to the cross-flow component. However, embodiments also include the gas inlet 14 being above the gas outlet 13 and the relative flows of the gas and sorbent through the reactor comprising a co-flow component in addition to the cross-flow component.

As shown in Figure 3, the first and second manifold are preferably arranged to connect to every second gas duct. Embodiments also include manifolds being connected to both sides of the reactor, such that every gas duct is connected to manifolds for gas supply and extraction. This is particularly appropriate when the gas flows are large as it reduces the gas flow velocity into and out of the reactor.

Embodiments include there being any number of baffles 2 in each gas duct. For example, the number of baffles 2 in a gas duct may be between one and ten. The superficial cross flow velocity of gas through the reactor depends on the vertical spacing of the baffles 2 in gas duct and so the number and spacing of the baffles 2 is preferably designed so that an appropriate cross-flow velocity is achieved for the expected operating conditions of the reactor.

In use, solid particles/pellets of sorbent are fed into the inlet 5 at the top of the reactor. A carbonaceous gas, such as a flue gas, is fed into the gas inlet 14. The sorbent moves through the upper bed 6 and is split so that it travels into the plurality of parallel moving beds 7. The baffle plates 2 in the gas ducts 4 force the gas to make a plurality of flows through one or more the moving beds 7. In each moving bed 7, the relative flow between the solid and the gas has both a cross-flow component, due to the gas moving across the moving bed 7, and a vertical component, which is either a counter-flow or co-flow relative to that of the sorbent.

An advantage of the reactor design according to embodiments is that the contact between the gas and sorbent is very effective. The gas is forced to make a plurality of crossings of one or more moving beds 7 as the gas flows from the gas inlet to the gas outlet. This is clearly shown in Figure 6 in which there is a cross-flow and counter-flow of gas and sorbent. The reactor has approximate properties to those of a counter flow moving bed reactor in which the solids are distributed over a very large area and there is a low bed thickness. The narrow thickness of the bed allows the gas cross-flow velocity to be low and the gas pressure drop is consequently also low.

Another advantage of the reactor according to embodiments is that the volume of the gas ducts is a lot larger than that of the moving beds 7. For example, the width of each of the gas ducts may be in the range of 10cm to 100 cm, whereas the width of each moving bed 7 may be in the range 1cm to 10 cm. Even when the volume ratio of the gas and solid is greater than a thousand, the reactor can be easily designed to accommodate gas flow velocities in the preferred range of lOm/s to 20 m/s, and sorbent velocities in the moving beds 7 that are in the range lcm/s to 10 cm/s.

The reactor according to embodiments has the combined advantages of a fluidized bed reactor’s large gas flow capacity and a moving bed reactor’s specific retention time of a solid sorbent. The mechanical stress on sorbent particles/pellets is also low due to the low velocity of the sorbent through the moving beds 7.

Figure 6 shows gas flow paths 18 between adjacent gas ducts 4 via a moving bed 7. The average vertical gas flow velocity, Ufg, is 19. The superficial cross-flow velocity of the gas, Ucf, is 20. The moving bed velocity, Umb, is 17.

The cooling requirements of the cooling tubes are explained further below.

The specific surface area of randomly packed spheres of diameter 0.75 mm is 4800 m²/m³. The total bulk volume of the solid sorbent pellets in a reactor, for a CaO looping CCS system for flue gas from a 400 MW class NGCC, is 48 m³. The results in a total heat surface area between solid sorbent and flue gas of 230000 m². For a flue gas passing such a bed at a superficial velocity of approximately 1 m/s, the heat exchange coefficient will be in the order of 500 W/m²K. The required heat removal in such a system in order to keep the temperature of the solid sorbent constant is 150 MW. This implies a temperature difference between gas and solid sorbent of

150 MW

500 W m~²K~¹ 230,000 m²

1.5K

For a flow velocity of gas in the ducts of 10-20 m/s, the heat exchange coefficients are 300-500 W/m²K. This is for forced convection over typical tube bundles with tube diameter, D, being 20mm - 50 mm. The specific area of a 20 mm diameter tube in a rectangular array with pitch of 2xD is 40 m². Embodiments include this being increased by use of finned tubes. The required volume of the gas ducts in the carbonator reactor will be 50 times larger than the volume of the solids beds of 48 m³. This gives following required temperature difference between flue gas and tube wall:

150 MW __________________________________— 3

500 W m^K^-1 40 m²m~³ 50 48 m³

This implies that gas will be efficiently cooled each time it passes the heat exchanger installed in the gas ducts. Therefore, the solid sorbent in the moving bed 7 will be cooled indirectly in an efficient manner. As the number of passes increases, the temperature rise in the gas phase per pass of a moving bed 7 will reduce.

Figure 8 shows a CCS system according to an embodiment. A solid sorbent, such as CaO particles/pellets, as described throughout the present document, are transferred around the CCS system in a loop. The CCS system is appropriate for retro-fitting to a fossil fuel power plant.

The CCS system comprises an input of flue gas 801, a carbonator 803, a calcinator 809, a riser 821, and output of cleaned flue gas 818 and a separate output of substantially pure CO₂.

The carbonator is preferably a gas-solid reactor as described throughout the present document with reference to Figure 1 to 7F. In the carbonator 803, sorbent reacts with the flue gas to thereby substantially reduce the CO2 concentration in the flue gas. In the calcinator 809, the sorbent is regenerated by heating the sorbent so that it releases CO2. The riser then return the regenerated sorbent to the input of the carbonator 803.

Embodiments include a number of modifications and variations to the above-described techniques.

Embodiments are not restricted to the use of a solid sorbent and the solid used in a gassolid reactor according to embodiments may be any type of solid reactant.

Embodiments are not restricted to the use of a specific reactor and the reactor according to embodiments may be any type of mass transfer system.

The openings in the sidewalls of each moving bed are all preferably less than 500pm, more preferably less than 400pm and further preferably less than 200pm.

The reactor according to embodiments can be made with a wide range of dimensions depending on the application. The walls of lower bed 9 of the reactor are preferably sloped at an angle between about 60 and 70 degrees in order to facilitate the movement of sorbent out of the reactor due to gravity.

Embodiments include the lower bed 9 comprising one or more space consuming structures that may be hollow. These assist the movement of the sorbent out of the reactor.

The baffle plates 2 are preferably substantially rigid so that they help to strengthen the structure of the reactor, in particular the walls of the moving beds 7.

The temperature at which the reaction between the sorbent and the gas occurs is dependent on the application. For a CCS system in which a flue gas is reacted with a sorbent, a temperature of about 600°C is appropriate.

Throughout embodiments the use of moving beds is described. The moving beds according to embodiments are generally mass transfer regions.

Embodiments have been described with reference to a solid sorbent based on CaO. However, embodiments include the reactor being used with other types of solid sorbent for use in CCS.

Embodiments include the gas-solid reactor being used in other applications than CCS. In particular, the sorbent may, for example, be a sorbent of SO2 or other gasses.

Although embodiments have been presented with the gas to be cleaned being flue gas, embodiments may be used with any gas and are not restricted to being a flue gas from a combustion process. The gas to be cleaned may be referred to as a dirty gas. The dirty gas may be sour gas directly output from a well head. The sour gas would be cleaned by capturing the hydrogen sulphide content.

Embodiments are appropriate for industrial scale processes. In particular, embodiments are particularly appropriate for providing a gas capture system that captures carbon dioxide gas generated by a power station. This includes all types of power station that generate carbon dioxide gas. All of the components of the gas capture system of embodiments are scalable such that the gas capture system is suitable for both capturing gas from the power stations that are the largest generators of carbon dioxide gas as well a power stations that are relatively small generators of carbon dioxide gas.

The flow charts and descriptions thereof herein should not be understood to prescribe a fixed order of performing the method steps described therein. Rather, the method steps may be performed in any order that is practicable. Although the present invention has been described in connection with specific exemplary embodiments, it should be understood that various changes, substitutions, and alterations apparent to those skilled in the art can be made to the disclosed embodiments without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims

1. A mass transfer system for providing mass transfer between a gas and a solid reactant, the mass transfer system comprising:

a gas inlet arranged to receive a gas flow into the mass transfer system;

a gas outlet arranged to provide a gas flow out of the mass transfer system;

a reactant inlet arranged to receive an input of solid reactant into the mass transfer system;

a reactant outlet arranged to provide an output of solid reactant from the mass transfer system;

one or more mass transfer regions arranged between the reactant inlet and the reactant outlet such that, in use, the solid reactant is retained within the one or more mass transfer regions as the solid reactant moves through the mass transfer regions and the mass transfer between the gas and the solid reactant occurs in the one or more mass transfer regions;

a first gas chamber; and a second gas chamber, that is different from the first gas chamber;

wherein the first gas chamber, second gas chamber and one or more mass transfer regions are arranged such that, in use, there is a flow path for gas from the gas inlet to the gas outlet that comprises gas flowing from the first gas chamber into one of the one or more mass transfer regions, the gas then flowing from said one of the mass transfer regions into the second gas chamber and the gas then flowing from the second gas chamber back into said one of the mass transfer regions.

2. The mass transfer system according to claim 1, wherein the first gas chamber, second gas chamber and said one of the one or more mass transfer regions are arranged such that, in use, the gas flow from the first gas chamber into said one of the one or more mass transfer regions is a direct gas flow; and the gas flow from said one of the one or more mass transfer regions into the second gas chamber is a direct gas flow.

3. The mass transfer system according to claim 1 or 2, wherein there are a plurality of mass transfer regions.

4. The mass transfer system according to claim 3, wherein the number of mass transfer regions is between 3 and 20; the number of mass transfer regions is preferably 10.

5. The mass transfer system according to any preceding claim, further comprising:

an upper bed for the solid reactant;

a main body that comprises the one or more mass transfer regions; and an lower bed for the solid reactant;

wherein:

the upper bed is arranged between the reactant inlet and the main body; and the lower bed is arranged between the main body and the reactant outlet.

6. The mass transfer system according to claim 5, wherein:

the main body is substantially cuboid; and each of the one or more mass transfer regions is substantially cuboid.

7. The mass transfer system according to any preceding claim, wherein each of the one or more mass transfer regions is arranged such that, in use, solid reactant moves vertically downwards through the mass transfer region.

8. The mass transfer system according to any preceding claim, wherein each mass transfer region comprises sidewalls that each separate the mass transfer region from a gas chamber.

9. The mass transfer system according to claim 8, wherein each of the sidewalls is configured such that, in use, gas is able to flow through the sidewall and substantially no solid reactant can pass through the sidewall.

10. The mass transfer system according to claim 8 or 9, wherein:

each sidewall is a plate that comprises openings; and the diameter of the openings in the plate is 400 pm or less.

11. The mass transfer system according to any preceding claim, wherein:

one or more gas ducts are provided between each two adjacent mass transfer regions and/or between a mass transfer region and an outer wall of the main transfer system;

each gas duct comprises a plurality of gas chambers, wherein the gas chambers in each gas duct are separated by one or more baffle plates that gas is unable to flow through.

12. The mass transfer system according to claim 11, wherein the gas chambers in each gas duct are aligned vertically with adjacent gas chambers separated by a horizontal baffle plate.

13. The mass transfer system according to claim 11 or 12, wherein each baffle plate is arranged either between the side walls of two mass transfer regions or between the sidewall of a mass transfer region and an outer wall of the mass transfer system.

14. The mass transfer system according to any of claims 11 to 13, wherein there is at least one baffle pate in each gas duct.

15. The mass transfer system according to any of claims 11 to 14, wherein the number of baffle plates in each gas duct is between 2 and 5.

16. The mass transfer system according to any of claims 11 to 15, wherein the baffle plates are arranged such that, in use, gas is arranged to flow across one or more mass regions at least three times when the gas flows from the gas inlet to the gas outlet.

17. The mass transfer system according to any preceding claim, wherein the mass transfer system comprises cooling tubes in one or more of the gas chambers; and in use, the cooling tubes are arranged to cool gas in the gas chambers.

18. The mass transfer system according to any preceding claim, further comprising a flow control mechanism at an end of each mass transfer region for controlling the rate at which solid reactant can move through the mass transfer region.

19. The mass transfer system according to claim 18, wherein the flow control mechanism is a loop seal.

20. The mass transfer system according to claim 18 or 19, wherein each flow control mechanism comprises an gas inlet; and in use, some of the gas is fed into the gas inlet of each flow control mechanism in order to control the rate at which the solid reactant moves through the one or more mass transfer regions.

21. The mass transfer system according to any preceding claim, wherein:

the reactant inlet is arranged above the reactant outlet; and the gas inlet is arranged below the a gas outlet such that, in use, there is a counter flow of gas and solid reactant through the one or more mass transfer regions.

22. The mass transfer system according to any of claims 1 to 20, wherein:

the reactant inlet is arranged above the reactant outlet; and the gas inlet is arranged above the a gas outlet such that, in use, there is a co-flow of gas and solid reactant through the one or more mass transfer regions.

23. The mass transfer system according to any preceding claim, wherein the solid reactant is a sorbent.

24. The mass transfer system according to claim 23, wherein the sorbent is a sorbent of carbon dioxide gas.

25. A carbon capture and storage, CCS, system comprising the mass transfer system according to any preceding claim.

26. A method of mass transfer between a gas and a solid reactant in a mass transfer system:

providing a flow path of a solid reactant through one or more mass transfer regions of the mass transfer system, wherein the solid reactant is retained within the one or more mass transfer regions as the solid reactant moves through the one or more mass transfer regions and the mass transfer between a gas and the solid reactant

5 occurs in the one or more mass transfer regions; and providing a flow path of the gas through the mass transfer system such that the gas flows from a first gas chamber of the mass transfer system into one of the one or more mass transfer regions, the gas then flows from said one of the mass transfer

10 regions into a second gas chamber of the mass transfer system, that is different from the first gas chamber, and the gas then flows from the second gas chamber back into said one of the mass transfer regions.

27. The method according to claim 26, wherein the mass transfer system is a mass

15 transfer system according to any of claims 1 to 24.